1. Field of the Invention
The present invention relates generally to hearing prostheses, and more particularly, to measurements in a hearing prosthesis.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive or sensorineural. In many people who are profoundly deaf, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence, destruction or damage to the hairs in the cochlea which transduce acoustic signals into nerve impulses. Various hearing prostheses have been developed to provide individuals suffering from sensorineural hearing loss with the ability to perceive sound.
One type of hearing prosthesis, referred to as a cochlear implant system, includes an electrode assembly implanted in the cochlea. Electrical stimulation signals are delivered directly to the auditory nerve via the electrode assembly, thereby inducing a hearing sensation in the implant recipient.
Conductive hearing loss occurs when the normal mechanical pathways which conduct sound to the cochlea are impeded. This problem may arise, for example, as a result of damage to the ossicular chain or ear canal. Individuals suffering from conductive hearing loss frequently retain some form of residual hearing because the hairs in the cochlea are often undamaged. For this reason, individuals who suffer from conductive hearing loss typically are not candidates for a conventional cochlear implant system because insertion of the electrode assembly into the cochlea may severely damage or destroy the remaining hairs in the cochlea.
Individuals suffering from conductive hearing loss typically receive an acoustic hearing aid. Hearing aids receive ambient sound, amplify the sound, and direct the amplified sound through the ear canal. The amplified sound reaches the cochlea and causes motion of the cochlea fluid, thereby stimulating the hairs in the cochlea.
Unfortunately, hearing aids do not benefit all individuals suffering from conductive hearing loss. For example, some individuals are prone to chronic inflammation or infection of the ear canal. Other individuals have malformed or absent outer ear and/or ear canals as a result of a birth defect, or as a result of common medical conditions such as Treacher Collins syndrome or Microtia.
Individuals unable to benefit from hearing aids may benefit from implantable hearing prostheses that deliver mechanical energy to the recipient. In one type of implantable hearing prosthesis, an implanted actuator is rigidly connected to the ossicular chain, thereby enabling direct vibration of the ossicular chain to induce an auditory response. In another type of hearing prosthesis, an implanted actuator is rigidly connected to the cochlea and operates by directly vibrating the perilymph in the inner ear. Both of these types of hearing prostheses often require complicated surgery, and they are not well-suited for implantation into growing children because of the rigid connections between the actuator and the ossicular chain and perilymph, respectively.
Another type of hearing prosthesis, referred to as a bone conduction device, such as a Baha®, has an actuator implanted into the skull bone of the recipient. The actuator provides vibrations directly to the recipient's skull bone. These vibrations are conducted by the recipient's bony structure to the inner ear to elicit an auditory response.